The present invention relates to a new and improved construction of a thread grinding machine for grinding short threads by means of a plunge-cut technique and for face or cone grinding, wherein the to-and-fro movement of the tool headstock needed for thread grinding is generated by a threaded rod and a lever, there also being provided a coupling which enables interrupting this movement for the face and cone grinding, so that the tool headstock remains stationary in its position, the coupling and its control being structured such that the coupling and uncoupling always is carried out in the same exact position of both the workpiece headstock and also the workpiece spindle.
With the heretofore known thread grinding machines one is basically concerned with so-called universal grinding machines, the basic design of which is such that there can be ground threads or screws having single groove disks. Grinding of the threads is accomplished in the same manner as is conventional when cutting threads or screws upon a lathe. The grinding disk is brought into the desired position before the screw or thread to be ground and then travels through the thread, frequently pre-cut, until reaching the end thereof where it then lifts-off of the thread or screw. The direction of rotation of the workpiece is now reversed and the grinding disk returns without performing any grinding action. For the next pass the disk now is further infed or advanced by a certain amount than was the case during the last or preceding pass, the rotational direction of the workpiece again reversed and the disk again passes through the threading until reaching the end thereof, where the disk is lifted-off. These operations repeat until the thread has reached the desired depth.
It should be readily apparent that the amount of time needed for producing a thread or screw according to this technique is extremely high. The amount by which the grinding disk is infed or advanced during each pass is small and the rotational speed of the workpiece exceedingly small, so that quite a bit of time is consumed until there is performed a grinding operation during only one pass. Since there are required at least as many revolutions as there are pitches of the thread, the grinding time is long. A further loss in time prevails by virtue of the fact that the grinding disk returns without exerting any grinding action.
The manufacturers of such grinding machines then decided to construct plunge-cut grinding devices as an auxiliary device. During plunge-cut grinding the grinding disk is grooved in accordance with the thread to be ground and is wider than the latter, so that for accomplishing one pass theoretically there is required only one revolution of the workpiece. In practice, however, the workpiece rotates at somewhat more than one revolution.
Of course, the workpiece must carry out a to-and-fro movement, corresponding to the desired thread pitch. With the state-of-the-art thread grinding machines this is achieved by moving the workpiece headstock by means of a lead or guide spindle and change gears. These change gears must be exchanged when altering the pitch which is to be ground. For grinding all conventional pitches there is thus needed a large number of change gears, and it is also to be appreciated that the exchange operation is time consuming.
Additionally, there is also known a machine where the threading is ground during forward and rearward rotations. The to-and-fro movement is produced in that the workpiece spindle externally carries a threading which travels in a stationary nut and this spindle is driven by a motor and a rotational speed-varying device, the motor being reversed in polarity during each forward and reverse movement. This machine is associated with the considerable drawback that for each thread to be ground there must be available a special spindle with associated nut. This solution is exceedingly expensive, since, to avoid errors in the pitch by play and wear, there must be advantageously employed cone revolving spindles or planetary spindles.
Now oftentimes it is required of parts which are provided with threading, that at least there be available a surface or a cone, for instance, in the case of collets or chucks of automatic lathes and the like, which have as low as possible eccentric or concentricity error in relation to the threading. It is therefore advantageous to grind threading and flat surfaces or cones in a clamping device. However, this is not possible when working with either of the two above-described methods, since in this case it is necessary to interrupt the to-and-fro movement of the workpiece. Since both methods are not capable of grinding in the aforedescribed manner, when working with parts having external threading one has resorted to the technique of grinding the threading and other surfaces of the workpiece which should possess only slight concentricity errors in relation to the threading, between tips, and wherein the centers advantageously previously likewise have been ground upon a special machine. In the case of internal threads it is oftentimes necessary to grind a centering diameter at the workpiece prior to thread grinding. For the thread grinding and the inner cone grinding to be accomplished at another machine or also hole grinding, the workpiece is then clamped at this diameter. In both instances both with internal thread grinding as well as also external thread grinding the concentricity error is greater than when the thread and flat surfaces or cone are ground in a clamping device.